This is transcribed from a support call

I connect a Siemens IM300 meter to pnpscada through its internal modem. The modem connects. But in Communication Monitor, the server asks, and the meter never answers back.

Does the communication packets start and end with 7E?

Yes

Then the problem is that when you added the meter, you chose the HDLC sub-protocol, instead of the TCP sub protocol.

The DLMS meter protocol supports 2 sub-protocols that are incompatible with each other.

In the Siemens build Wasion meters, the internal GPRS modem talks the TCP sub protocol, and in the Netvendor build Wasion meters, the internal GPRS modem talks the HDLC sub protocol.

Delete the meter, and add it again. You'll see in the Add dialog, there is a drop-list where you can choose the sub-protocol. Choose the TCP sub-protocol this time, not the HDLC sub-protocol.